1. Field of Invention
This invention relates to a novel electrophotographic apparatus and process suitable for use in electrophotography and, more specifically, to a developing unit and a method for controlling the consistency of density in an electrophotographic process.
2. Background
It is often useful to print large quantities of multi-colored prints to paper for the purpose of disseminating multiple copies of reports or brochure information. One objective of this kind of printing is that all the reports or brochures look the same, which means that all the printing of the color and monochrome pages must maintain a consistent density as printing progresses. It is not desirable to allow the densities of primary colors to vary from page to page because the final product of the reports and/or brochures will be degraded if the colors are varying from document to document. Therefore it is important to measure and control the density of images (i.e., plated toner or ink) during the printing process to assist in maintaining constant density during the printing process.
To accomplish the printing of constant density images over time in the printing process or other electrophotographic applications, several methods have been described. One attempt disclosed in U.S. Pat. No. 5,243,391 (Williams) is a system that measures the percent solids in the ink solution as an electrical resistance and then adjusts the gap between the developing element and the ink receptor to modify the electric field in the printing nip. This kind of hardware is both costly and difficult to maintain in the liquid ink environment.
Another example of an image control system is in U.S. Pat. No. 5,933,685 (Yoo) which uses the detection of ink solids by optical means. No provision is made for detecting ink conductivity. However, constant density printing can occur with this arrangement only if the ink conductivity remains constant in the presence of decreasing ink solids and ink conductivity is not considered by this process. A similar method also uses ink concentration sensing for print density control but also fails to account for ink conductivity variations that may affect print density.
Many attempts (for example, U.S. Pat. No. 4,468,112 to Suzuki) are found that try to overcome the above defined problem of image density variation other than by sensing the toner concentration control in the developing unit. These methods of print density control need a test patch (i.e., reference image on a patch) to be prepared separately from an output image, the density of the reference image which has been developed is then measured, and the toner is supplied such that its density assumes a prescribed value. In this method, since in many cases an-electrostatic image of the reference patch is always developed under constant potential contrast, the fact that the density of the patch assumes a prescribed value means that the ink concentration is variably controlled so that the toner charge amount is maintained at a constant level. These attempts also further require a density measuring system to measure the density of the test patch. All such similar methods require recording, developing and measuring steps that may add cost and complexity to the printing hardware. Another similar approach (e.g., U.S. Pat. No. 6,115,561 to Fukushima) uses a special pattern in the imaging system along with a lookup table, but the density measurement of the special pattern is still required or else the measurement needs more than just one special pattern. Clearly, the previous methods for print density control with respect to time all need special hardware in addition to the printing hardware, and many also need the involvement of the ink receptor where test patches must be printed and analyzed.
One method as disclosed in, for example, Japanese unexamined Patent Publication Nos. 108070/1989, 314268/1989, 8873/1990, 110476/1990, 75675/1991, and 284776/1991, is the use of a pixel counting method wherein the image density of an output image or the number of pixels that are written is counted, and the amount of toner consumption is estimated in a corresponding manner so as to supply the toner. This is a method in which the amount of toner that to be consumed for forming a dot is assumed. With this method, there has been the problem that even if the toner supply error may be very small in each print, the errors accumulate over a long term, leading to a large toner concentration error in the final run.